2013 AMC 10B Problem 22
Below is the professionally curated solution for Problem 22 of the 2013 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2013 AMC 10B solutions, or check the answer key.
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Difficulty rating: 2060
22.
The regular octagon has its center at Each of the vertices and the center are to be associated with one of the digits through with each digit used once, in such a way that the sums of the numbers on the lines and are all equal. In how many ways can this be done?
Solution:
Let be defined as:
This means that From here, let's assume We will see that the other cases are similar enough to omit.
If then we know that the pairs of numbers that satisfy the equality above are: There are ways to distribute the pairs over the four groups, and then ways for these groups to swap elements (i.e. ).
Now, if we look at the and cases, we see a similar pattern in the number of groupings and swaps. As such, we have: possibilities.
Thus, the correct answer is C .
Problem 22 in Other Years
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